Semiconductor device having doped epitaxial region and its methods of fabrication

ABSTRACT

Embodiments of the present invention describe a epitaxial region on a semiconductor device. In one embodiment, the epitaxial region is deposited onto a substrate via cyclical deposition-etch process. Cavities created underneath the spacer during the cyclical deposition-etch process are backfilled by an epitaxial cap layer. The epitaxial region and epitaxial cap layer improves electron mobility at the channel region, reduces short channel effects and decreases parasitic resistance.

This is a Divisional application of Ser. No. 12/643,912 filed Dec. 21,2009, which is presently pending.

BACKGROUND

1. Field

The present invention relates to the field of semiconductor processingand more particularly to a semiconductor device having doped epitaxialregions and its methods of fabrication.

2. Discussion of Related Art

Increasing the performance of semiconductor devices, in particulartransistors, has always been a major consideration in the semiconductorindustry. For example, during the design and fabrication of metal oxidesemiconductor field effect transistors (MOSFETs), it has always been acommon goal to increase the electron mobility of the channel region andto reduce the parasitic resistance to improve device performance.

Other methods of improving device performance include, for example,reducing the overall resistance of the MOSFET by doping the regionbetween the source/drain regions and the channel region, which isreferred to as the ‘tip’ or source/drain extension regions of theMOSFET. For example, a dopant is implanted in the source/drain regionsand an annealing step diffuses the dopant towards the channel region.However, there are limits to controlling the dopant concentration andlocation. Furthermore, the implant and diffusion method does not addressthe issues of lateral undercut or parasitic resistance at the tipregions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view that illustrates a semiconductor devicein accordance with one embodiment of the present invention.

FIG. 2 is a cross-sectional view that illustrates a semiconductor devicein accordance with another embodiment of the present invention.

FIG. 3 is a cross-sectional view that illustrates a semiconductor devicein accordance with another embodiment of the present invention.

FIG. 4 is a perspective view that illustrates a semiconductor device inaccordance with another embodiment of the present invention.

FIGS. 5A-5F are cross-sectional views that illustrate the method offabricating the semiconductor device shown in FIG. 1.

FIGS. 6A-6F are cross-sectional views that illustrate the method offabricating the semiconductor device shown in FIG. 2.

FIGS. 7A-7C are cross-sectional views that illustrate the method offabricating the semiconductor device shown in FIG. 3.

FIGS. 8A-8I are perspective views that illustrate the method offabricating the semiconductor device shown in FIG. 4.

FIGS. 9-15 are cross-sectional views of the semiconductor device shownin FIGS. 8E-8I.

FIG. 8E′ is a perspective view that illustrates an alternativeembodiment of the semiconductor device shown in FIG. 8E.

FIG. 9′ is a perspective view that illustrates an alternative embodimentof the semiconductor device shown in FIG. 9.

DETAILED DESCRIPTION

A semiconductor device having doped epitaxial regions and its method offabrication are described. In the following description, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. In other instances, well knownsemiconductor processing techniques and features have not been describedin particular detail in order not to unnecessarily obscure the presentinvention.

Embodiments of the present invention describe a method of formingepitaxial regions on a semiconductor device. In one embodiment, theepitaxial region is an in-situ carbon and phosphorus doped silicon(Si_(y)(C,P)_(1-y)) region deposited via cyclical deposition-etchprocess. Cavities created underneath the spacer during the cyclicaldeposition-etch process are backfilled by a very highly phosphorus dopedsilicon (Si_(y)P_(1-y)) epitaxial cap layer. The fabrication of theepitaxial region and cap layer stack in a Self-aligned Epi Tip (SET)architecture provides a dramatic transistor performance gain due tocombined effects of increased electron mobility gain at channel region,reduced short channel effects (due to carbon suppressing phosphorusdiffusion), and reduced parasitic resistance provided by very highphosphorus doping in the (Si_(y)P_(1-y)) epitaxial cap layer.

FIG. 1 illustrates a cross-sectional view of a semiconductor device inaccordance with one embodiment of the present invention. Thesemiconductor device comprises a substrate 200 made of a semiconductormaterial such as, but not limited to, monocrystalline silicon. In oneembodiment, the substrate 200 is the silicon film of a silicon oninsulator (SOI) substrate, or a multi-layered substrate comprisingsilicon, silicon germanium, germanium, III-V compound semiconductors.

A gate dielectric 310 is formed over a channel region of the substrate200. In one embodiment, the dielectric layer 310 is made from any wellknown insulative material, such as but not limited to silicon oxide(e.g., SiO₂). In another embodiment, the dielectric layer 310 is made ofa high-k dielectric material having a substantially higher dielectricconstant than silicon dioxide (i.e. k>3.9). Examples of high-kdielectric materials include but are not limited to tantalum oxide(Ta₂O₅), titanium oxide (TiO₂) and hafnium oxide (HfO₂).

A gate electrode 320 is formed over the gate dielectric 310. In oneembodiment, the gate electrode 320 is made of any well known materials,such as but not limited to polysilicon. In other embodiments, the gateelectrode 320 is made of a metal or metal alloy material such as, butnot limited to, platinum, tungsten or titanium.

In one embodiment, a hard mask 410 is formed on top of the gateelectrode 320. In one embodiment, hard mask 410 is made of a materialsuch as but not limited to silicon nitride or silicon oxynitride.Spacers 420, 440 are formed on opposite sidewalls of the gate electrode320. In one embodiment, spacers 420, 440 are formed along the entiresidewall width of the gate electrode 320. Spacers 420, 440 comprisesidewalls 421, 441, and bottom surfaces 422, 442. In one embodiment, thespacers 420, 440 are made of a material such as but not limited to asilicon nitride, silicon dioxide or silicon oxynitride.

In an embodiment of the present invention, a recessed source interface220 and a recessed drain interface 230 are formed on the substrate 200at opposite sides of the gate electrode 320. In one embodiment, aportion of the recessed source interface 220 extends laterally beneaththe bottom surface 422 of the spacer 420 and beneath a portion of thegate electrode 320. Similarly, a portion of the recessed drain interface230 extends laterally beneath the bottom surface 442 of the spacer 440and beneath a portion of the gate electrode 320.

A source region 501 is formed over the recessed source interface 220. Inan embodiment of the present invention, the source region 501 comprisesan epitaxial region 531 formed over the recessed source interface 220. Acap layer 541 is formed over the epitaxial region 531. The source region501 comprises a source epi-tip region 503 that includes portions of theepitaxial region 531 and cap layer 541 formed directly beneath thespacer 420 and gate dielectric 310.

A drain region 502 is formed over the recessed drain interface 230. Inone embodiment, the drain region 502 comprises an epitaxial region 532formed over the recessed drain interface 230. A cap layer 542 is formedover the epitaxial region 532. The drain region 502 comprises a drainepi-tip region 504 that includes portions of the epitaxial region 532and cap layer 542 formed directly beneath the spacer 440 and gatedielectric 310. By forming the source and drain epi-tip regions 503, 504in relatively close proximity to the channel region, a largerhydrostatic stress is induced on the channel region, resulting in higherelectron mobility and increasing drive current.

In an embodiment of the present invention, the epitaxial regions 531,532 comprise silicon and carbon doped with phosphorus. In this case, thesemiconductor device shown in FIG. 1 is a NMOS planar or trigatetransistor with a Self-aligned Epi Tip (SET) architecture. In oneembodiment, the epitaxial regions 531, 532 comprise silicon having acarbon concentration of about 0.5 atomic % to 4 atomic % and aphosphorus concentration of about 9E19 cm⁻³ to 3E21 cm⁻³. In a specificembodiment, the epitaxial regions 531, 532 comprise silicon having acarbon concentration of 2.2 atomic % and a phosphorus concentration of2E20 cm⁻³. The substitutional carbon (over 2 atomic %) in the epitaxialregions 531, 532 of the source and drain regions 501, 502 imparthydrostatic stress on the channel region, which enhances electronmobility. Furthermore, the substitutional carbon suppresses anyphosphorus diffusion during any subsequent thermal anneals, thusreducing short channel effects.

In an embodiment of the present invention, the cap layers 541, 542 areepitaxial layers comprising silicon doped with phosphorus. In oneembodiment, the cap layers 541, 542 comprise silicon having a phosphorusconcentration of about 8E19 cm⁻³ to 3E21 cm⁻³. In a specific embodiment,the cap layers 541, 542 comprise silicon having a phosphorusconcentration of 2E21 cm⁻³. The high phosphorus concentration level inthe cap layers 541, 542 reduces parasitic resistance, particularly incontact resistance between salicide and source/drain regions 501, 502.

FIG. 2 illustrates a cross-sectional view of a semiconductor devicesimilar to FIG. 1. The substrate 200 is made of {001} silicon, andcomprises a recessed source interface 240 having a {111} facet 241 inthe {111} crystallographic plane of the {001} silicon substrate 200, anda recessed drain interface 250 having a {111} facet 251 in the {111}crystallographic plane of the {001} silicon substrate 200. The {111}facets 241, 251 provide reduced volume in depletion and correspondingimproved control of short channel effects. In one embodiment, therecessed source and drain interfaces 240, 250 each further comprises a{010} facet 242, 252 in the {010} crystallographic plane of the {001}silicon substrate 200, where the {010} facets 242, 252 extend directlybeneath the gate electrode 320. The {010} facets 242, 252 contribute tomore precisely defining the metallurgical channel length of thesemiconductor device and reduce short channel effects.

Similar to FIG. 1, the semiconductor device shown in FIG. 2 comprises asource region 501 and a drain region 502, each having an epitaxialregion 531, 532 and a cap layer 541, 542. Epitaxial regions 531, 532 andcap layers 541, 542 are formed over the recessed source and draininterfaces 240, 250 including their {111} facets 241, 251 and {010}facets 242, 252. The source region 501 comprises a source epi-tip region505 that includes portions of the epitaxial region 531 and cap layer 541surrounded by the spacer 420, the gate dielectric 310 and the {111},{010} facets 241, 242. The drain region 502 comprises a drain epi-tipregion 506 that includes portions of the epitaxial region 532 and caplayer 541 surrounded by the spacer 440, the gate dielectric 310 and the{111}, {010} facets 251, 252. Forming the source and drain epi-tipregions 505, 506 in relatively close proximity to the channel regioninduces a larger hydrostatic stress on the channel region, thusincreasing electron mobility which results in higher drive current.

FIG. 3 illustrates a cross-sectional view of a semiconductor devicesimilar to FIG. 2. In one embodiment, the source and drain regions 501,502 each comprises an epitaxial layer 610, 620 formed over the recessedsource and drain interfaces 240, 250 including their {111} facets 241,251 and {010} facets 242, 252.

The source region 501 comprises a source epi-tip region 611 thatincludes portions of the epitaxial layer 610 surrounded by the spacer420, the gate dielectric 310 and the {111}, {010} facets 241, 242. Thedrain region comprises a drain epi-tip region 621 that includes portionsof the epitaxial layer 610 surrounded by the spacer 440, the gatedielectric 310 and the {111}, {010} facets 251, 252. Forming the sourceand drain epi-tip regions 611, 621 in relatively close proximity to thechannel region induces a larger hydrostatic stress on the channelregion, thus increasing electron mobility which results in higher drivecurrent.

In an embodiment of the present invention, the epitaxial layer 610, 620comprises silicon doped with phosphorus. In one embodiment, theepitaxial layers 610, 620 comprise silicon having a phosphorusconcentration of about 8E19 cm⁻³ to 3E21 cm⁻³. In a specific embodiment,the epitaxial layers 610, 620 comprise silicon having a phosphorusconcentration of 2E21 cm⁻³. The high phosphorus concentration level inthe epitaxial layers 610, 620 reduces parasitic resistance, particularlyin contact resistance between salicide and source/drain regions 501,502.

FIGS. 1, 2 and 3 illustrate the application of epitaxial regions inplanar transistors to enhance electron mobility at the channel region orto reduce contact resistance at the source/drain regions. It can beappreciated that the epitaxial regions are not limited to planartransistors but can be fabricated on other devices, such as but notlimited to a tri-gate transistor. FIG. 4 illustrates a perspective viewof a tri-gate device comprising a substrate 200 having a semiconductorbody or fin 260 (represented by dashed lines). A gate electrode 340 isformed over 3 surfaces of the fin 260 to form 3 gates. A hard mask 410is formed on top of the gate electrode 340. Gate spacers 460, 470 areformed at opposite sidewalls of the gate electrode 340. The sourceregion comprises the epitaxial region 531 formed on a recessed sourceinterface 266 and on a fin 260 sidewall. The cap layer 541 is depositedover the epitaxial region 531.

FIGS. 5A-5F illustrate a method of forming the semiconductor device asdiscussed in relation to FIG. 1. The fabrication of the semiconductordevice begins by providing the substrate 200 as shown in FIG. 5A. Thegate dielectric 310 is formed over a desired channel region of thesubstrate 200. In one embodiment, the gate dielectric 310 is formed byany well known methods, such as but not limited to physical vapordeposition (PVD), chemical vapor deposition (CVD) or atomic layerdeposition (ALD).

The gate electrode 320 is formed over the gate dielectric 310. In anembodiment of the present invention, the gate electrode 320 is asacrificial gate electrode that is subsequently replaced by an actualgate electrode in a replacement gate process. The hard mask 410 isformed on top of the gate electrode 320. In an embodiment of the presentinvention, gate electrode 320 and hard mask 410 are deposited using PVDor CVD, and then patterned using well known photolithography and etchingtechniques.

Spacers 420, 440 are then formed on opposite sidewalls of the gateelectrode 320. Spacers 420, 440 comprises sidewalls 421, 441, and bottomsurfaces 422, 442 that are formed on the top surface of the substrate200. In one embodiment, the spacers 420, 440 are formed by using wellknown techniques, such as depositing a layer of spacer material over theentire substrate 200 including the gate electrode 320, and thenanisotropically etching the layer of spacer material to form the spacers420, 440 on the sidewalls of gate electrode 320.

Next, a source region and a drain region are formed on the substrate200. In an embodiment of the present invention, fabrication of thesource and drain regions begins by recessing portions of the substrate200 using well known etching techniques, such as but not limited to dryetching or wet etching. In an embodiment of the present invention, a wetetching comprising an etchant chemistry that is substantially selectiveto the substrate 200 is utilized to recess the substrate 200 so as toform a recessed source interface 220 and a recessed drain interface 230as shown in FIG. 5B.

In one embodiment, the wet etching undercuts the spacers 420, 440 andforms a source epi-tip cavity 271 between the bottom surface 422 ofspacer 420 and the recessed source interface 220, and forms a drainepi-tip cavity 272 between the bottom surface of spacer 440 and therecessed drain interface 230. As a result, the source epi-tip cavity 271and drain epi-tip cavity 272 expose the bottom surfaces 422, 442 of thespacers 420, 440. In one embodiment, the source epi-tip cavity 271 anddrain epi-tip cavity 272 also expose portions of the gate dielectric310. As a result, a portion of the recessed source interface 220 extendslaterally beneath the spacer 420 and beneath a portion of the gateelectrode 320. Similarly, a portion of the recessed drain interface 230extends laterally beneath the spacer 440 and beneath a portion of thegate electrode 320.

It can be appreciated that the wet etching can be controlled (e.g. byadjusting the etching duration) so that the source and drain epi-tipcavities 271, 272 do not expose the gate dielectric 310. For instance,the recessed source interface 220 only extends laterally beneath thespacer 420, and the recessed drain interface 230 only extends laterallybeneath the spacer 440.

In an embodiment of the present invention, the recessed source and draininterfaces 220, 230 define the channel region of the semiconductordevice. The channel region refers to the portion of the substrate 200located directly beneath the gate dielectric 310 and between therecessed source and drain interfaces 220, 230.

Next, an epitaxial region is deposited over each of the recessed sourceand drain interfaces 220, 230 by alternatingly exposing the substrate200 to a first precursor and a second precursor. Fabrication of theepitaxial region begins, in FIG. 5C, by exposing the entire substrate200 to the first precursor so as to deposit epitaxial films 511, 512 onthe recessed source and drain interfaces 220, 230. In the case where thesubstrate 200 is made of monocrystalline silicon, the recessed sourceand drain interfaces 220, 230 are monocrystalline surfaces that allowepitaxial growth of the epitaxial films 511, 512 thereon. On the otherhand, the hard mask 410, the spacers 420, 440, and the gate dielectric310 are non-crystalline surfaces. As a result, an amorphous layer 513 isdeposited on the top surface of the hard mask 410, on the sidewalls 421,441 and bottom surfaces 422, 442 of the spacers 420, 440, and onportions of the bottom surface of the gate dielectric 310.

In an embodiment of the present invention, the first precursor comprisesa silicon-containing compound, a carbon-containing compound, and adopant. In one embodiment, the silicon-containing compound includes, butnot limited to, silanes and halogenated silanes. Such silicon-containingcompound includes silane (SiH₄), disilane (Si₂H₆), trisilane (Si₃H₈),dichlorosilane (SiH₂Cl₂), and penta-chloro silane.

In one embodiment, the carbon-containing compound includes, but notlimited to, organosilanes. For example, the carbon-containing compoundcomprises mono-methyl silane (CH₃—SiH₃). In one embodiment, thecarbon-containing compound is mixed with hydrogen (H₂) or argon. Forexample, mono-methyl silane (CH₃—SiH₃) is mixed with hydrogen (H₂) orargon with the CH₃—SiH₃ concentration in the range of 0.5% to 20%.

In an embodiment of the present invention, the dopant is a n-type dopantsuch as, but not limited to, phosphorus or arsenic. In one embodiment,the Phosphorus dopant is incorporated in the epitaxial film usingphosphine (PH₃) without any dilution in Hydrogen or an inert gas, suchas N₂ or Ar. In another embodiment, the phosphine gas is mixed withhydrogen, for example as a mixture of 3% phosphine (PH₃) in hydrogen(H₂).

In one embodiment, the first precursor is delivered or released onto thesubstrate 200 with a carrier gas. In one embodiment, the carrier gasincludes, but is not limited, to hydrogen (H₂), or any inert gas such asnitrogen (N₂) argon and helium and any combinations thereof.

In an embodiment of the present invention, the substrate 200 is exposedto the first precursor at a temperature of about 500 to 700 degreesCelsius, at a pressure of about 5 to 300 torr, and for a time durationof about 3 to 60 seconds. In a specific embodiment, the substrate 200 isexposed to the first precursor at a temperature of 600 degrees Celsius,at a pressure of 30 torr, and for a time duration of 15 seconds.

In one embodiment, the epitaxial films 511, 512 are grown to have athickness of about 6 to 100 Angstroms. In a specific embodiment, theepitaxial films 511, 512 are grown to have a thickness of 50 Angstroms.In the case where the first precursor uses a phosphorus dopant, thedeposited epitaxial films 511, 512 are crystalline films containingsilicon and carbon doped with phosphorus (i.e. in-situ carbon andphosphorus doped silicon layer). The amorphous layer 513 containssilicon and carbon doped with phosphorus.

An optional surface pre-treatment can be performed on the substrate 200before exposing it to the first precursor to facilitate epitaxial growthand reduce surface defects. In an embodiment of the present invention,the surface pre-treatment includes a hydrogen bake treatment performedon the substrate 200 (in FIG. 5B) to clean the recessed source and draininterfaces 220, 230. The hydrogen bake treatment desorbs oxygen andrenders surface reconstruction so that the epitaxial films 511, 512 canreadily nucleate without formation of defects. In one embodiment, thehydrogen bake treatment is performed at around 700 to 1050 degreesCelsius for a duration of about 10 to 120 seconds. In an embodiment ofthe present invention, hydrogen chloride (HCl) is added to the hydrogenbake treatment. The hydrogen chloride (HCl) enables removal of about 1to 3 monolayers of the recessed source and drain interfaces 220, 230 sothat they are free of oxygen, hydrocarbons and any other contaminants.In one embodiment, the hydrogen bake treatment with hydrogen chloride(HCl) is performed at a lower temperature of around 700 to 900 degreesCelsius for a duration of about 10 to 120 seconds. Alternatively,chlorine (Cl₂), germane (GeH₄) or phosphine (PH₃) can be used as anadditional or alternative chemical compound for hydrogen chloride (HCl).

In an alternative embodiment, the surface pre-treatment utilizes an etchstep to clean the recessed source and drain interfaces 220, 230. In oneembodiment, the etch step uses an etchant gas, such as but not limitedto hydrogen (H₂), anhydrous hydrochloric acid (HCl), or a mixture ofgermane (GeH₄) and hydrogen (H₂). In another embodiment, the surfacepre-treatment uses a combination of the etch step and the hydrogen baketreatment.

Before exposing the substrate 200 to the second precursor, a purgingprocess can be performed to remove the first precursor and otherby-products from the substrate 200. In one embodiment, the purgingprocess injects an inert gas, such as but not limited to nitrogen (N₂),helium or argon to remove any unreacted first precursor or by-products.

Next, in FIG. 5D, the entire substrate 200 is exposed to the secondprecursor to remove the amorphous layer 513 from the sidewalls 421, 441and bottom surfaces 422, 442 of the spacers 420, 440. Furthermore, thesecond precursor also removes any amorphous layer 513 formed on the hardmask 410 and beneath the gate dielectric 310. In one embodiment, thesecond precursor 900 uses an etchant chemistry that etches the amorphouslayer 513 faster than the epitaxial films 511, 512. In one embodiment,the second precursor 900 is an etchant gas, such as but not limited tohydrogen (H₂), anhydrous hydrochloric acid (HCl), and a mixture ofgermane (GeH₄) and hydrogen (H₂). Germane (GeH₄) enables etch throughcatalysis, thus providing high etch rate at low temperatures.

In one embodiment, the substrate 200 is exposed to the second precursorat a pressure of about 30 to 300 torr, and for a time duration of about5 to 60 seconds. In a specific embodiment, the substrate 200 is exposedto the second precursor at a pressure of 80 torr, and for a timeduration of 20 seconds. In one embodiment, the temperature is maintainedat substantially the same level when exposing the substrate 200 to boththe first precursor and the second precursor.

Due to the weak chemical bonding between the amorphous layer 513 and thehard mask 410, spacers 420, 440 and gate dielectric 310, the secondprecursor easily removes the amorphous layer 513 deposited thereon. Thesecond precursor reacts with the amorphous layer 513 to convert it intoby-products, thus removing the amorphous layer 513 from the hard mask410, spacers 420, 440 and gate dielectric 310.

On the other hand, the epitaxial films 511, 512 have strong chemicalbonds with the recessed source and drain interfaces 220, 230. Due to thestrong chemical bonds, only small portions of the epitaxial films 511,512 are removed by the second precursor. In one embodiment, thethickness of the epitaxial films 511, 512 deposited during FIG. 5C orthe duration of exposing the second precursor to the substrate 200 inFIG. 5D can be adjusted so as to effectively remove the amorphous layer513 while maintaining sufficient thickness for the epitaxial films 511,512.

FIGS. 5C and 5D illustrate one deposition-etch cycle of forming theepitaxial films 511, 512 over the recessed source and drain interfaces220, 230. In one embodiment, the deposition-etch cycle is repeated usingthe same type of first and second precursors until a desired number ofepitaxial films are deposited. For example, FIG. 5E shows epitaxialregions 531, 532 containing ten epitaxial films each.

It can be appreciated that the epitaxial regions 531, 532 are notlimited to only ten layers of epitaxial films each. In one embodiment,about 3 to 100 deposition-etch cycles are performed to form theepitaxial regions 531, 532. In a specific embodiment, 30 deposition-etchcycles are performed to form the epitaxial regions 531, 532 having athickness of around 30 nanometers.

In an embodiment of the present invention, the epitaxial regions 531,532 are deposited with a graded concentration of carbon or phosphorus.The carbon and phosphorus concentration of each epitaxial filmdeposition can be optimized to provide optimal selectivity anddefect-free epitaxy. Furthermore, the graded carbon or phosphorusconcentration promotes the removal of the amorphous material during thedeposition-etch cycles. In one embodiment, the graded carbonconcentration of the epitaxial regions 531, 532 (shown in FIG. 5E)begins at around 0.5 atomic % for the lowest epitaxial film and isgradually increased to a desired level of about 2 atomic % in theuppermost epitaxial film. In another embodiment, the graded phosphorusconcentration level of the epitaxial regions 531, 532 begins at around8E19 cm⁻³ for the lowest epitaxial film and is gradually increased to adesired level of around 2E21 cm⁻³ in the uppermost epitaxial film. Inone embodiment, the epitaxial regions 531, 532 are deposited with acombination of graded carbon concentration (0.5-2 atomic %) and gradedphosphorus concentration (8E19-2E21 cm⁻³).

The epitaxial regions 531, 532 are selectively formed over the recessedsource and drain interfaces 220, 230 as shown in FIG. 5E. However, theremoval of the amorphous layer 513 during each deposition-etch cycleresults in voids or cavities 281,282 formed between the bottom surfaces422, 442 of spacers 420, 440 and the top surfaces of the epitaxialregions 531, 532. In one embodiment, the cavities 281, 282 also extendbetween portions of the gate dielectric 310 and the epitaxial regions531, 532. The cavities 281, 282 may cause detrimental effects on thetransistor performance, and thus need to be eliminated. In an embodimentof the present invention, the cavities 281, 282 are substantiallybackfilled by cap layers 541, 542 selectively deposited over theepitaxial regions 531, 532 as shown in FIG. 5F.

In an embodiment of the present invention, the cap layers 541, 542 areselectively deposited over the epitaxial regions 531, 532 in a singledeposition process by exposing the substrate 200 to a third precursor.In one embodiment, the third precursor comprises the samesilicon-containing compound and dopant of the first precursor, and thesame etchant gas of the second precursor.

In the case where the epitaxial regions 531, 532 are crystalline filmshaving silicon and carbon doped with phosphorus, the third precursoruses the same phosphorus dopant to form cap layers 541, 542. Thecrystalline surfaces of the epitaxial layers 531, 532 allow epitaxialgrowth of cap layers 541, 542 thereon, and as a result, the cap layers541, 542 are epitaxial layers containing silicon doped with phosphorus.Apart from backfilling the cavities, the phosphorus doped silicon caplayers 541, 542 provides an advantage of inducing tensile stress on thechannel region, thereby increasing electron mobility and improving thedevice performance.

In one embodiment, a co-flown deposition technique is used to expose thesubstrate 200 to the silicon-containing compound, the dopant, and theetchant gas at the same time. In one embodiment, the etchant gas doesnot include germane (GeH₄). During deposition, the etchant gas easilyremoves any silicon and phosphorus-containing compound that are weaklybonded on the hard mask 410 and spacers 420, 440 so that the cap layers541, 542 are deposited over the epitaxial regions 531, 532 and notdeposited on the hard mask 410 or the spacers 420, 440.

In an embodiment of the present invention, the substrate 200 is exposedto the third precursor at a temperature of about 550 to 800 degreesCelsius, at a pressure of about 10 torr to atmospheric pressure, and fora time duration of about 30 to 900 seconds. In a specific embodiment,the substrate 200 is exposed to the first precursor at a temperature of635 degrees Celsius, at a pressure of 600 torr, and for a time durationof 180 seconds. In one embodiment, the cap layers 541, 542 are grown tohave a thickness of about 50 to 500 Angstroms. In a specific embodiment,the cap layers 541, 542 are grown to have a thickness of 300 Angstroms.

Portions of the epitaxial region 531 and cap layer 541 directly beneaththe spacer 420 and gate dielectric 310 forms the source epi-tip region503. Similarly, portions of the epitaxial region 532 and cap layer 542directly beneath the spacer 440 and gate dielectric 310 forms the drainepi-tip region 504. By forming the source and drain epi-tip regions 503,504 in relatively close proximity to the channel region, a largerhydrostatic stress is induced on the channel region, resulting in higherelectron mobility and increasing drive current. The stress can befurther amplified by increasing the carbon concentrations of the sourceand drain epi-tip regions 503, 504 during fabrication of the epitaxialregions 531, 532. Furthermore, the carbon concentrations of the sourceand drain epi-tip regions 503,504 also help to suppress any phosphorusdiffusion during subsequent thermal anneals.

In an embodiment of the present invention, the gate electrode 320 is asacrificial gate electrode that is subsequently replaced by an actualgate electrode in a replacement gate process. In one embodiment, thereplacement gate process begins by depositing a mask on the cap layers541, 542 and then planarizing the mask to be coplanar with the hard mask410 (not shown). Next, the hard mask 410 and gate electrode 320 areremoved using well known etching techniques. After removing the hardmask 410 and gate electrode 320, the actual gate electrode is thendeposited on the gate dielectric 310. In one embodiment, the actual gateelectrode is a metal gate electrode comprising materials such as, butnot limited to, platinum, tungsten or titanium. This completes thefabrication of the semiconductor device shown in FIG. 1.

FIGS. 6A-6F illustrate a method of forming the semiconductor device asdiscussed in relation to FIG. 2. The fabrication of the semiconductordevice begins by providing the substrate 200 as shown in FIG. 6A. Thesemiconductor device shown in FIG. 6A is the same as FIG. 5A, and thuswill not be discussed in detail. Briefly, the semiconductor devicecomprises the gate dielectric 310 formed over a desired channel regionof the substrate 200. The gate electrode 320 is formed over the gatedielectric 310. In an embodiment of the present invention, the gateelectrode 320 is a sacrificial gate electrode that is subsequentlyreplaced by an actual gate electrode in a replacement gate process. Hardmask 410 is formed on top of the gate electrode and spacers 420, 440 areformed at the sidewalls of the gate electrode 320.

Next, a source region and a drain region are formed on the substrate200. In an embodiment of the present invention, fabrication of thesource and drain regions begins by recessing portions of the substrate200 using well known etching techniques, such as but not limited to dryetching or wet etching. In an embodiment of the present invention, a wetetching that is substantially selective to the substrate 200 is utilizedto recess the substrate 200 so as to form a recessed source interface240 and a recessed drain interface 250 as shown in FIG. 6B.

In an embodiment of the present invention, the substrate 200 is made of{001} silicon. The wet etch uses an etchant chemistry that etches the{001} silicon substrate 200 based on crystallographic direction, and inparticular etches the {001} silicon substrate 200 much more slowly alongits {111} crystallographic plane to form the {111} facets 241, 251 asthe etch proceeds much more rapidly in other crystallographicdirections. As a result, a source epi-tip cavity 271 is formed betweenthe bottom surface 422 of spacer 420 and the {111} facet 241. A drainepi-tip cavity 272 is formed between the bottom surface of spacer 440and the {111} facet 251.

The wet etch chemistry includes, but not limited to, an ammonia-based oramine-based etchant. Examples of ammonia-based etchants are ammoniumhydroxide (NH4OH), tetramethylammonium hydroxide (TMAH) andbenzyltrimethylammonium hydroxide (BTMH). The wet etch chemistryincludes other types of etchants, such as potassium hydroxide (KOH) andsodium hydroxide (NaOH).

In one embodiment, the wet etch further creates {010} facets 242, 252 inthe channel region of the {001} silicon substrate 200. The {010} facets242, 252 extends directly beneath the gate dielectric 310. In a specificembodiment, {010} facets 242, 252 are formed up to a length of around 3nanometers from the gate dielectric 310.

Next, an epitaxial region is deposited over each of the recessed sourceand drain interfaces 240, 250 by alternatingly exposing the substrate200 to a first precursor and a second precursor. The method offabricating the epitaxial region, as illustrated in FIGS. 6C, 6D and 6E,is similar to the methods of fabrication discussed in FIGS. 5C, 5D and5E. Before exposing the substrate 200 to the first precursor, anoptional surface pre-treatment can be performed on the substrate 200 tofacilitate epitaxial growth and reduce surface defects. In oneembodiment, the surface pre-treatment comprises a hydrogen baketreatment and/or etching step as previously discussed in FIG. 5C toclean the recessed source and drain interfaces 240, 250.

Beginning from FIG. 6C, the entire substrate 200 is exposed to the firstprecursor so at to deposit epitaxial films 511, 512 on the recessedsource and drain interfaces 240, 250. The recessed source and draininterfaces 240, 250, including their {111} facets 241, 251 and {010}facets 242, 252, are monocrystalline surfaces that allow epitaxialgrowth of the epitaxial films 511, 512 thereon. On the other hand, thehard mask 410, the spacers 420, 440, and the gate dielectric 310 arenon-crystalline surfaces, and thus the amorphous layer 513 is depositedthereon. The same first precursor and process conditions, as discussedin relation to FIG. 5C, are applicable here and will not be discussed.

Next, in FIG. 6D, the entire substrate 200 is similarly exposed to thesecond precursor to remove the amorphous layer 513 from the sidewalls421, 441 and bottom surfaces 422, 442 of the spacers 420, 440.Furthermore, the second precursor also removes any amorphous layer 513formed on the hard mask 410 and beneath the gate dielectric 310. Thesame second precursor and process conditions, as discussed in relationto FIG. 5D, are applicable here and will not be discussed.

FIGS. 6C and 6D illustrate one deposition-etch cycle of forming theepitaxial films 511, 512 over the recessed source and drain interfaces240, 250 including their {111} facets 241, 251 and {010} facets 242,252. The deposition-etch cycle is repeated until a desired number ofepitaxial films are deposited. For illustration purposes, FIG. 6E showsepitaxial regions 531, 532 containing ten epitaxial films each. In anembodiment of the present invention, the epitaxial regions 531, 532 aredeposited with a graded concentration of carbon or phosphorus aspreviously described in FIG. 5E. For example, the epitaxial regions 531,532 (shown in FIG. 6E) are deposited with a graded carbon concentrationof around 0.5 atomic % for the lowest epitaxial film and are graduallyincreased to a desired level of about 2 atomic % for the uppermostepitaxial film. Alternatively, the epitaxial regions 531, 532 aredeposited with a graded phosphorus concentration level of around 8E19cm⁻³ for the lowest epitaxial film and are gradually increased to adesired level of around 2E21 cm⁻³ for the uppermost epitaxial film. Inone embodiment, the epitaxial regions 531, 532 are deposited with acombination of graded carbon concentration (0.5-2 atomic %) and gradedphosphorus concentration (8E19-2E21 cm⁻³).

The removal of the amorphous layer 513 during each deposition-etch cyclesimilarly results in cavities 281,282 formed between the bottom surfaces422, 442 of spacers 420, 440 and the top surfaces of the epitaxialregions 531, 532. The cavities 281, 282 are substantially backfilled bythe cap layers 541, 542 selectively deposited over the epitaxial regions531, 532 as shown in FIG. 6F.

In one embodiment, the cap layers 541, 542 are selectively depositedover the epitaxial regions 531, 532 in a single deposition process byexposing the substrate 200 to a third precursor. The same thirdprecursor and process conditions, as discussed in relation to FIG. 5F,are applicable here. In the case where the epitaxial regions 531, 532are crystalline films having silicon and carbon doped with phosphorus,the third precursor uses the same phosphorus dopant to form cap layers541, 542. The crystalline surfaces of the epitaxial regions 531, 532allow epitaxial growth of cap layers 541, 542 thereon, and as a result,the cap layers 541, 542 are epitaxial layers containing silicon dopedwith phosphorus. This completes the fabrication of the semiconductordevice shown in FIG. 2.

FIGS. 7A-7C illustrate a method of forming the semiconductor device asdiscussed in relation to FIG. 3. Beginning from FIG. 7A, the fabricationof the semiconductor device begins by providing the substrate 200. Thesemiconductor device shown in FIG. 7A is the same as FIG. 5A, and thuswill not be discussed here.

Next, a source region and a drain region are formed on the substrate200. In an embodiment of the present invention, fabrication of thesource and drain regions begins by recessing portions of the substrate200 using well known etching techniques, such as but not limited to dryetching or wet etching. In one embodiment, the wet etching used in FIG.6B is similarly applied here to recess the substrate 200 so as to form arecessed source interface 240 and a recessed drain interface 250, asshown in FIG. 7B. The wet etch uses the same etchant chemistry asdescribed in relation to FIG. 6B to form {111} facets 241, 251 in the{111} crystallographic plane of the {001} silicon substrate 200. In oneembodiment, the wet etch further creates {010} facets 242, 252 in thechannel region of the {001} silicon substrate 200.

Next, epitaxial layers 610, 620 are selectively deposited over therecessed source and drain interfaces 240, 250 as shown in FIG. 7C. In anembodiment of the present invention, the epitaxial layers 610, 620 areselectively deposited in a single deposition process by exposing thesubstrate 200 to a precursor comprising an etchant gas.

In one embodiment, the precursor comprises the silicon-containingcompound and dopant similarly described in FIG. 5C. In one embodiment,the silicon-containing compound includes, but not limited to, silanesand halogenated silanes. Such silicon-containing compound includessilane (SiH₄), disilane (Si₂H₆), trisilane (Si₃H₈), dichlorosilane(SiH₂Cl₂), and penta-chloro silane. In an embodiment of the presentinvention, the dopant is a n-type dopant such as, but not limited to,phosphorus or arsenic. In one embodiment, the phosphorus dopant isintroduced into the epitaxial layers using phosphine (PH₃) without anydilution in Hydrogen or an inert gas, such as N₂ or Ar. In anotherembodiment, the phosphine gas is mixed with hydrogen, for example as amixture of 3% phosphine (PH₃) in hydrogen (H₂). In one embodiment, theetchant gas of the precursor includes, but is not limited to hydrogen(H₂) and anhydrous hydrochloric acid (HCl).

In one embodiment, a co-flown deposition technique is used to deliverthe precursor, including the etchant gas to the substrate 200 at thesame time. In one embodiment, the substrate 200 is exposed to theprecursor at a temperature of about 550 to 800 degrees Celsius, at apressure of about 10 torr to atmospheric pressure, and for a timeduration of about 30 to 2000 seconds. In a specific embodiment, thesubstrate 200 is exposed to the first precursor at a temperature of 635degrees Celsius, at a pressure of 600 torr, and for a time duration of800 seconds.

In one embodiment, the epitaxial layers 610, 620 are grown to have athickness of about 30 to 2000 Angstroms. In a specific embodiment, theepitaxial layers 610, 620 are grown to have a thickness of 750Angstroms. In the case where a phosphorus dopant is used, the epitaxiallayers 610, 620 comprise silicon doped with phosphorus.

In the case where the substrate 200 is made of monocrystalline silicon,the recessed source and drain interfaces 240, 250 including their {111}facets 241, 251 and {010} facets 242, 252 are monocrystalline surfacesthat allow epitaxial growth of the epitaxial layers 610, 620 thereon.Since the hard mask 410 and spacers 420, 440 have non-crystallinesurfaces, the etchant gas easily removes any silicon andphosphorus-containing compound that are weakly bonded on the hard mask410 and spacers 420, 440 during deposition so that the epitaxial layers610, 620 are deposited over the recessed source and drain interfaces240, 250 and not deposited on the hard mask 410 or spacers 420, 440.

Portions of the epitaxial layer 610 deposited between the spacer 420 and{111}, {010} facets 241, 242 form the source epi-tip region 611.Similarly, portions of the epitaxial layer 620 deposited between thespacer 440 and {111}, {010} facets 251, 252 form the drain epi-tipregion 621. By forming the source and drain epi-tip regions 611, 621 inrelatively close proximity to the channel region, a larger hydrostaticstress is induced on the channel region, resulting in higher electronmobility. Furthermore, the phosphorus doped silicon epitaxial layers610, 620 induces tensile stress on the channel region, therebyincreasing electron mobility and improving the device performance. Thiscompletes the fabrication of the semiconductor device shown in FIG. 3.

In addition, an optional surface pre-treatment can be performed on thesubstrate 200 before exposing it to the precursor to facilitateepitaxial growth and reduce surface defects. For example, a similarhydrogen bake treatment described in relation to FIG. 5C is performed onthe substrate 200 (in FIG. 7B) to clean the recessed source and draininterfaces 240, 250 including theirs {111} facets 241, 251 and {010}facets 242, 252.

FIGS. 8A-8I illustrate a method of forming the tri-gate device asdiscussed in relation to FIG. 4. The fabrication of the tri-gate devicebegins by providing the substrate 200 as shown in FIG. 8A. The substrate200 comprises a semiconductor body or fin 260 extending from thesubstrate 200 through the isolation regions 710, 720. In one embodiment,the isolation regions 710, 720 are shallow trench isolation (STI)regions formed by common techniques, such as etching the substrate 200to form trenches, and then depositing oxide material onto the trenchesto form the STI regions. The isolation regions 710, 720 are made fromany well known insulative material, such as but not limited to siliconoxide (e.g., SiO₂).

In one embodiment, the fin 260 comprises a top surface 261 above theisolation regions 700. The fin 260 further includes a front surface 262exposed above the isolation region 710, and a back surface 263 exposedabove the isolation region 720. In one embodiment, the fin 260 is madefrom the same semiconductor materials as the substrate 200.

Next, in FIG. 8B, a gate dielectric 330 is formed over a portion of thetop surface 261, the front surface 262 and the back surface 263. In oneembodiment, the gate dielectric 330 is formed by any well known methods,such as but not limited to physical vapor deposition (PVD), chemicalvapor deposition (CVD) or atomic layer deposition (ALD).

Then, a gate electrode 340 is formed over the gate dielectric 330, andexposing portions 264, 265 of the fin 260 at either sides of the gateelectrode 340. In one embodiment, the gate electrode 340 is made of anywell known materials, such as but not limited to polysilicon. The gateelectrode 340 formed over the top surface 261, the front surface 262 andthe back surface 263 creates three gates for the tri-gate device. Thehard mask 410 is then formed on top of the gate electrode 320.

Next, gate spacers 460, 470 are deposited on opposite sidewalls of thegate electrode 340 as shown in FIG. 8C. In one embodiment, the spacers460, 470 are formed by using well known techniques, such as depositing alayer of spacer material over the entire substrate 200 including thegate electrode 320, and then anisotropically etching the layer of spacermaterial to form the spacers 460, 470 on the sidewalls of gate electrode340. At the same time, fin spacers 480, 490 are formed on the sidewallsof the exposed portions 264, 265 of the fin 260. In one embodiment, thegate spacers 460, 470 and fin spacers 480, 490 are made of a materialsuch as but not limited to a silicon nitride, silicon dioxide or siliconoxynitride.

Next, a source region and a drain region are formed on the substrate200. In an embodiment of the present invention, fabrication of thesource and drain regions begins in FIG. 8D by removing the fin spacers480, 490 from the sidewalls of the exposed portions 264, 265 of the fin260. The fin spacers 480, 490 are removed by well known etchingtechniques, such as but not limited to dry etching or wet etching.

In one embodiment, an anisotropic wet-etch is used to completely removethe fin spacers 480, 490 from the exposed portions 264, 265 of the fin260. At the same time, the anisotropic wet-etch also removes portions ofthe gate spacers 460, 470, thus exposing portions of the hard mask 410sidewalls. Since the gate spacers 460, 470 have a larger height andthickness than the fin spacers 480, 490, the anisotropic wet-etchremoves the fin spacers 480, 490 faster than the gate spacers 460, 470.The anisotropic wet-etch can be controlled to completely remove the finspacers 480, 490 but leaving sufficient thickness of the gate spacers460, 470 on the gate electrode 340 so that the gate electrode 340sidewalls are not exposed.

Next, an etching is performed on the substrate 200 to recess the exposedportions 264, 265 of the fin 260. In an embodiment of the presentinvention, the etching uses an etchant chemistry that is substantiallyselective to the fin 260 to recess the exposed portion 264 so as to forma recessed source interface 266 below the top surface of the isolationregions 710, 720, and to form a fin sidewall 267 as shown in FIG. 8E. Onthe other side of the gate electrode 340, the exposed portion 264 isrecessed to form a recessed drain interface 268 and a fin sidewall 269.In one embodiment, the recessed source and drain interfaces 266, 268 areabout 100 to 400 Angstroms below the top surface of the isolationregions 710, 720.

FIG. 9 illustrates a cross-sectional view of the tri-gate device showingthe fin sidewall 267 extending from the top surface 261 to the recessedsource interface 266, and the fin sidewall 269 extending from the topsurface 261 to the recessed drain interface 268. In an embodiment of thepresent invention, the fin sidewalls 267, 269 are substantially coplanaror flushed with the gate spacers 460, 470 sidewalls 461, 471. In oneembodiment, the fin sidewalls 267, 269 are {110} facets in the {110}crystallographic plane of the substrate 200, and the recessed source anddrain interfaces 266, 268 are {100} facets in the {100} crystallographicplane of the substrate 200.

In an alternative embodiment, an isotropic etch is used to form finsidewalls 267, 269 recessed within the gate spacers 460, 470. FIG. 8E′is a perspective view of the tri-gate device showing the fin sidewall267 recessed within the gate spacer 470. FIG. 9′ is a cross-sectionalview showing both the fin sidewalls 267, 269 recessed beneath the gatespacer 460, 470. In one embodiment, the fin sidewalls 267, 269 arerecessed until about 25 to 200 Angstroms from the gate spacer sidewalls461, 471.

Continuing from FIG. 8E, an epitaxial region is then deposited over eachof the recessed source and drain interfaces 266, 268 by alternatinglyexposing the substrate 200 to a first precursor and a second precursor.The method of fabricating the epitaxial region, as illustrated in FIGS.8F, 8G and 8H, is similar to the methods of fabrication discussed inFIGS. 5C, 5D and 5E.

Beginning from FIG. 8F, the entire substrate 200 is exposed to the firstprecursor so at to deposit an epitaxial film 511 on the recessed sourceinterface 266 and fin sidewall 267. At the same time, an epitaxial film512 is deposited on the recessed drain interface 268 and fin sidewall269 as shown in cross-sectional view of FIG. 10. The recessed source anddrain interfaces 266, 268, and the fin sidewalls 267, 269, aremonocrystalline surfaces that allow epitaxial growth of the epitaxialfilms 511, 512 thereon. On the other hand, the hard mask 410, the gatespacers 460, 470, and isolation regions 710, 720 are non-crystallinesurfaces, and thus the amorphous layer 513 is formed thereon. The samefirst precursor and process conditions, as discussed in relation to FIG.5C, are applicable here and will not be discussed.

Next, in FIG. 8G, the entire substrate 200 is similarly exposed to thesecond precursor to remove the amorphous layer 513 from the gate spacers460, 470, and isolation regions 710, 720. Furthermore, the secondprecursor also removes any amorphous layer 513 formed on the hard mask410. FIG. 11 shows a cross-sectional view of the tri-gate device afterthe amorphous layer 513 has been removed. The same second precursor andprocess conditions, as discussed in relation to FIG. 5D, are applicablehere and will not be discussed.

FIGS. 8F-8G and FIGS. 10-11 illustrate one deposition-etch cycle offorming the epitaxial films 511, 512 over the recessed source and draininterfaces 266, 268 and the fin sidewalls 267, 269. The deposition-etchcycle is repeated until a desired number of epitaxial films aredeposited. In one embodiment, the final epitaxial regions 531, 532comprise five epitaxial films as shown in FIG. 12. In an embodiment ofthe present invention, the epitaxial regions 531, 532 are deposited witha graded concentration of carbon or phosphorus as previously describedin FIG. 5E. For example, the epitaxial regions 531, 532 (shown in FIG.12) are deposited with a graded carbon concentration of around 0.5atomic % for the lowest epitaxial film and are gradually increased to adesired level of about 2 atomic % for the uppermost epitaxial film.Alternatively, the epitaxial regions 531, 532 are deposited with agraded phosphorus concentration level of around 8E19 cm⁻³ for the lowestepitaxial film and are gradually increased to a desired level of around2E21 cm⁻³ for the uppermost epitaxial film. In one embodiment, theepitaxial regions 531, 532 are deposited with a combination of gradedcarbon concentration (0.5-2 atomic %) and graded phosphorusconcentration (8E19-2E21 cm⁻³)

In the alternative embodiment where the fin sidewalls 267, 269 arerecessed within the gate spacers 460, 470, the epitaxial regions 531,531 are formed in closer proximity to the channel region of the tri-gatedevice, thus inducing a higher amount of stress on the channel region.

The removal of the amorphous layer 513 during each deposition-etch cyclesimilarly results in voids or cavities 281,282 formed between theepitaxial regions 531, 532 and the isolation regions 710, 720 as shownin FIGS. 8H and 13. The cavities 281, 282 are substantially backfilledby the cap layers 541, 542 selectively deposited over the epitaxialregions 531, 532 as shown in FIGS. 8I, 14 and 15.

In one embodiment, the cap layers 541, 542 are selectively depositedover the epitaxial regions 531, 532 in a single deposition process byexposing the substrate 200 to a third precursor. The same thirdprecursor and process conditions, as discussed in relation to FIG. 5F,are applicable here. In the case where the epitaxial regions 531, 532are crystalline films having silicon and carbon doped with phosphorus,the third precursor uses the same phosphorus dopant to form cap layers541, 542. The crystalline surfaces of the epitaxial regions 531, 532allow epitaxial growth of cap layers 541, 542 thereon, and as a result,the cap layers 541, 542 are epitaxial layers containing silicon dopedwith phosphorus. The phosphorus doped silicon cap layers 541, 542provide an advantage of inducing tensile stress on the channel region ofthe semiconductor fin 260, which increases the electron mobility andimproves the device performance. This completes the fabrication of thesemiconductor device shown in FIG. 4.

Several embodiments of the invention have thus been described. However,those ordinarily skilled in the art will recognize that the invention isnot limited to the embodiments described, but can be practiced withmodification and alteration within the spirit and scope of the appendedclaims that follow.

We claim:
 1. A semiconductor device comprising: a substrate comprising agate electrode formed over a channel region of the substrate, and arecessed source interface and a recessed drain interface formed on thesubstrate at opposite sides of the gate electrode; a first spacer and asecond spacer formed on opposite sidewalls of the gate electrode,wherein a portion of the recessed source interface extends laterallybeneath the bottom surface of the first spacer, and a portion of therecessed drain interface extends laterally beneath the bottom surface ofthe second spacer; a source region comprising a first epitaxial regionformed over the recessed source interface, and a first cap layer formedover the first epitaxial region, wherein a portion of the first caplayer is formed between the first epitaxial region and the bottomsurface of the first spacer; and a drain region comprising a secondepitaxial region formed over the recessed drain interface, and a secondcap layer formed over the second epitaxial region, wherein a portion ofthe second cap layer is formed between the second epitaxial region andthe bottom surface of the second spacer.
 2. The semiconductor device ofclaim 1, wherein the first and second epitaxial regions each comprisessilicon and carbon doped with phosphorus.
 3. The semiconductor device ofclaim 2, wherein the wherein the first and second epitaxial regions eachcomprises silicon having a carbon concentration in the range of 0.5atomic % to 4 atomic %, and a phosphorus concentration in the range of9E19 cm⁻³ to 3E21 cm⁻³.
 4. The semiconductor device of claim 2, whereinthe first and second cap layers each comprises silicon doped withphosphorus.
 5. The semiconductor device of claim 4, wherein the firstand second cap layers each comprises silicon having a phosphorusconcentration in the range of 8E19 cm⁻³ to 3E21 cm⁻³.
 6. A semiconductordevice comprising: a substrate having an insulation layer formedthereon; a semiconductor body extending from the substrate through theinsulation layer, wherein the semiconductor body comprises a topsurface, a front surface and a back surface exposed above the insulationlayer, a first sidewall extending from the top surface to a sourceinterface, and a second sidewall opposite the first sidewall, the secondsidewall extending from the top surface to a drain interface; a gateelectrode formed over the top surface, the front surface and the backsurface of the semiconductor body; a first spacer and a second spacerformed on opposite sidewalls of the gate electrode; a source regioncomprising a first epitaxial region formed on the first sidewall and thesource interface, and a first cap layer formed over the first epitaxialregion, wherein a portion of the first cap layer is formed between thefirst epitaxial region and the insulation layer; and a drain regioncomprising a second epitaxial region formed over the second sidewall andthe drain interface, and a second cap layer formed over the secondepitaxial region, wherein a portion of the second cap layer is formedbetween the second epitaxial region and the insulation layer.
 7. Thesemiconductor device of claim 6, wherein the first sidewall issubstantially coplanar to the first spacer, and the second sidewall iscoplanar to the second spacer.
 8. The semiconductor device of claim 6,wherein the first sidewall is recessed within the first spacer, and thesecond sidewall is recessed within the second spacer.
 9. Thesemiconductor device of claim 6, wherein the source interface and thedrain interface are recessed below the top surface of the insulationlayer.
 10. The semiconductor device of claim 6, wherein the first andsecond epitaxial regions each comprises silicon and carbon doped withphosphorus.
 11. The semiconductor device of claim 10, wherein thewherein the first and second epitaxial regions each comprises siliconhaving a carbon concentration in the range of 0.5 atomic % to 4 atomic%, and a phosphorus concentration in the range of 9E19 cm⁻³ to 3E21cm⁻³.
 12. The semiconductor device of claim 6, wherein the first andsecond cap layers each comprises silicon doped with phosphorus.
 13. Thesemiconductor device of claim 12, wherein the first and second caplayers each comprises silicon having a phosphorus concentration in therange of 8E19 cm⁻³ to 3E21 cm⁻³.